In the production of cigarette filters, tow, such as cellulose acetate tow, which consists of a large number of crimped continuous filaments, is opened, is passed through a chamber in which plasticizer is applied to the tow, and thereafter is treated to reduce its cross-sectional size until it is approximately equal to the cross-sectional size of a cigarette. The condensed mass is formed into a coherent structure, typically by wrapping paper round it, and ultimately is cut into plugs of suitable length for incorporation directly into cigarettes or into a filter tip-applying machine.
One widely-used method of opening the tow consists of subjecting the tow, while being fed along a predetermined path, to a differential gripping action between a plurality of points spaced from one another transversely of the path so that certain laterally-spaced sections of the tow are positively gripped relative to other laterally-spaced sections of the tow. In this manner, there is produced, as a function of the differential positive gripping of the tow, a relative shifting of adjacent filaments longitudinally of the tow, whereby the crimps are moved out of registry with one another. The longitudinal relative displacement of the fibers usually is combined with a relative lateral displacement between adjacent filaments of the tow, whereby the combinations of the two relative filament movements brings about the complete opening of the tow.
This differential gripping action is accomplished by the provision, before the plasticizing chamber, of a pair of rollers, one of which is smooth surfaced and the other of which is grooved over its entire periphery. The tow is maintained under tension upstream of the differential gripping action so that after release of the tension on the downstream side of the differential gripping action, the tow blooms into a fluffy band which then passes through the plasticizer applying chamber, optionally after further lateral opening of the tow band, prior to feed to the filter rod-making machine.
Generally, the density of tow varies over a wide range and hence a feed of tensioned tow directly to the differential gripping rollers results in variations in weight per unit length of filter rod. This is undesirable since the resistance which filter rod sections including such variations offer to the passage of cigarette smoke varies, rendering inconsistent the draw characteristics of cigarettes to which filter tips formed from such filter rod sections are applied. It is usual, therefore, to attempt to control the density of the tow prior to formation of the filter rod.
It has previously been suggested to provide a filter rod making machine with pretensioning means to provide the tow to the differential gripping rollers at a substantially constant tension, and thereby decrease the difference between the densities of successive increments of the tow which subsequently is subjected to the stretching action. This pretensioning has been achieved by the use of a set of drag rollers upstream of the differential gripping rollers, the speed of rotation of the drag rollers being controlled by the speed of rotation of the differential gripping rollers and suitable braking means associated with the drag rollers. The braking is applied by the constant force applied by an upper roller against a lower roller having a compressible surface, thereby attempting to achieve a substantially even tension, and hence density, in the feed of the tow to the differential gripping rollers.
At high speeds of operation of filter rod-making machines, the response time of the constant braking action to variations in tow density is long, due at least in part to the inherent inertia of the pretensioning or drag rollers, causing variations in tow density fed to the differential gripping rollers, albeit variations which are less great than those in the original tow.
Such variations have led to sophisticated electronically-actuated control systems, such as are described in U.S. Pat. Nos. 3,741,846 and 3,613,975, to control the speed of drive motors for the pretensioning rollers to obtain a substantially even density feed of tow to the differential gripping rollers. However, such sophisticated control systems are complicated and expensive.